Wireless cellular communications is continuing to grow unabated. As wireless applications become increasingly widespread, the pressure on network operators to increase the capacity of their networks becomes more intense.
There are a number of ways of enhancing capacity in a wireless cellular network, including frequency hopping, micro cells, the introduction of adaptive antennas, and link adaptation. Link adaption has thus become an object of increasing interest in recent years.
In the following, conventional link adaptation mechanisms will be described for an exemplary wideband code division multiple access (WCDMA) scenario. A typical WCDMA scenario including two mobile devices (user equipment, UE), a base station (BS) communicating with the UEs, and a radio network controller (RNC) communicating with the BS is shown in FIG. 1. As can be seen from FIG. 1, WCDMA downlink transport channels to the UEs include a dedicated channel (DCH) and a high-speed downlink shared channel (HS-DSCH). The HS-DSCH is allocated to an UE on a time-slot by time-slot basis.
The basic link adaptation mechanisms in the WCDMA standard include power control on the DCH and adaptive coding and modulation on the HS-DSCH. Power control on DCH avoids allocating more power than is actually required to achieve a certain decoding quality to individual communications links. Since the total transmit power of the BS is limited, the implementation of such a power control scheme increases the network capacity. Additionally, avoiding excessively high power levels helps to reduce signal interference.
According to the link adaptation mechanism of adaptive coding and modulation, the transmission rate is adapted to the time-varying channel and interference conditions. In the case of favourable channel conditions for example, a larger modulation format or higher code rate is used to increase the data rate and thus enhance the network capacity.
A power control scheme in a WCDMA link adaptation context is for example described in H. Schotten and J. Röβler, “System Performance Gain in Interference Cancellation for WCDMA Dedicated and High-Speed Downlink Channels”, VTC 2002, Vancouver. The UE receiver configuration required to implement such a power control based link adaptation mechanism is depicted in FIG. 2 and will now be described in more detail.
A signal received from the BS by the UE receiver is demodulated, Rake combined and subjected to an interference cancellation step. Based on the signal that has been subjected to interference cancellation, an estimate for the signal-to-interference ratio (SIR) is determined and compared to a SIR target value. Depending on the result of this comparison, a power control algorithm generates a power up or a power down command for downlink that is sent in the uplink to the BS. Thus, a fast power control loop is established that allows adjustment of the power once per slot (at a rate of 1500 slots per second).
In addition to this fast power control loop an outer power control loop is provided. The outer power control loop adjusts the target SIR setpoint and aims at a constant frame error rate (FER). Outer loop control is based on a check of the cyclic redundancy code (CRC) that is obtained during decoding of a particular data frame. If for example the CRC check indicates that the transmission quality is decreasing, the SIR target may be increased and vice versa.
As has been mentioned above, adaptive coding and modulation is a further example of an efficient link adaptation mechanism. In FIG. 6, an approach for adaptive coding and modulation on a HS-DSCH known from H. Schotten and J. Röβler, “System Performance Gain in Interference Cancellation for WCDMA Dedicated and High-Speed Downlink Channels”, VTC 2002, Vancouver is depicted. In the scenario of FIG. 6, the transmission power is kept constant but the transmission rate is adapted to the current channel and interference conditions. A received signal that has been demodulated, Rake combined and subjected to interference cancellation is assessed to generate an estimate for the channel quality. This estimate is then used for channel quality indicator (CQI) signaling in uplink. The CQI signaling determines the modulation format and code rate that is used on downlink. By varying the modulation format and the code rate, the data rate on downlink can be adapted to the time-varying channel and interference conditions.
Efficient link adaptation requires a sufficiently accurate estimation of the quality of the received signal on the one hand and, to closely track channel and interference conditions, a low estimation and reporting delay of the signal quality on the other hand. These requirements are contradictory because depending on the implementation details of the receiver, a fast estimation of signal quality and a low reporting delay do often not allow a sufficiently accurate signal quality estimation.
There is thus a need for a method, a device and a system that enable a more efficient link adaptation based on a signal quality estimate.